1. Field of the Invention
This invention relates to a mechanical capacitor and, more particularly, to a mechanical capacitor which is used as a switch to turn on and off an acoustic ink jet ejector.
2. Description of Related Art
A single acoustic ink jet printhead ejector 100 is shown in FIG. 1. A channel forming layer 110 is formed on a substrate 102. An ink channel 112 is formed in the channel forming layer 110. A Fresnel lens 108 is formed on the surface of the substrate 102 in the ink channel 112. An opening 122 is formed on the top surface 120 of the channel forming layer 110. During normal operation, ink fills the ink channel 112 forming an ink free-surface 114. A piezoelectric device 31, positioned on the opposite side of the substrate 102 from the ink channel 112, comprises two electrodes 32 and 104 and a piezoelectric layer 106. When a radio-frequency (RF) signal is applied by RF power source 34 between the electrodes 32 and 104, the piezoelectric device 31 generates acoustic energy in the substrate 102 directed toward the ink channel 112. The Fresnel lens 108 focuses the acoustic energy entering the ink channel 112 from the substrate 102 onto the ink free-surface 114. The ink in the ink channel 112 forms an ink mound 116 in the ink-free surface 114. The ink mound 116 eventually becomes an ink drop 118 moving toward a recording medium.
In conventional acoustic ink jet printheads, a PIN diode controls ink ejection by switching the RF signal on and off. The RF signal powers the PIN diode and the piezoelectric device 31, which are serially connected. In this circuit, the PIN diode functions as a capacitor switch for the piezoelectric device. When the PIN diode capacitance is increased above a threshold by increasing a control voltage to the PIN diode, the piezoelectric device 31 activates, causing an ink drop 118 to be ejected from the ink channel 112.
Normally, an acoustic ink jet printhead contains an array of the ejectors 100. Because PIN diodes cannot be manufactured on the same substrate as the piezoelectric device 31, the PIN diodes are manufactured separately, placed onto the printhead substrate and electrically connected to the printhead by wire bonding. Thus, manufacturing conventional printheads not only incurs undesirable assembly costs, but also prevents manufacturing of high density ejector printheads, since space must be allowed for the manual diode assembly steps.